It's my understanding that it's essentially God. But I'm not really sure.
It's believed to be god. It does have the power to create those who rule over time, space and antimatter, though... So I guess you could get it to make you a Dialga, Palkia, and Giratina, then control the universe. Not a bad choice.
Dialga. Nothing better than controlling time itself, and flying. Of course, being able to ROAR your enemies into next week (literally!) does help. But then again Arceus would sort of have this power as well, but I'll go for something original.